Formula SAE Suspension
Overview
Northwestern’s Formula SAE racecar accelerating
Northwestern’s Formula SAE Racecar
Formula SAE (FSAE) is an intercollegiate engineering design competition organized by the Society of Automotive Engineers (SAE), where student teams design, build, and race formula-style cars. Each year, teams spend 8 to 12 months preparing for competition. The concept of the competition is for students to design, build, and race a prototype formula-style vehicle. The prototype is marketed toward the non-professional, weekend autocross racer. It must be high-performance, low-cost, easy to maintain, and suitable for manufacture at 1000 units per year. Students manage all aspects of the project like a small business, from the technical design to the management of an interdisciplinary team.
I was an active member of Northwestern’s Formula Racing Club for 2 years (2020-2022). I designed and manufactured two of the suspension components on the car, the rockers and the wheel hubs.
Introduction
Front (2 on the right) and Rear (2 on the left) Rockers
Front Hubs CAD
Rear Hubs CAD
The pictures above showcase my contributions to Northwestern’s Formula racecar. The hubs were manufactured out-of-house, but the rockers were machined by me.
The rockers transmit forces from the wheel to the Anti-Roll Bar (ARB), frame, tie rod, and shock. The configuration of the rockers is defined by the positions of the attachment points for the shock, pushrod, and ARB. The forces exerted on the rockers are derived from computations that factor in suspension geometry, vehicle mass, and anticipated loads during peak lateral and longitudinal accelerations.
Rockers Interfacing with Other Suspension Components
The hubs rigidly connect to the wheel assembly and allow, in conjunction with the uprights, the wheels to rotate with minimal friction. They also serve as a mounting point for the brake rotors via the use of float pins and, in the case of the rear hubs, transmit the driving force from the engine through the half-shafts into the wheels.
Goals
Rockers
Withstand the forces from the ARB and shock
Withstand non-planar forces (cause of failure in a previous year’s car)
Reduce weight
Hubs
Accommodate higher transmitted forces during corning, braking, and acceleration (previous year’s rear hubs sheared)
Reduce weight
Design
Rockers
The suspension geometry was provided by another member of the team, so I knew the attachment points for the frame, shocks, pushrods, and ARBs. The general procedure of designing the rockers was to start with a rectangular blank, cut out holes where attachments were made, perform a topology optimization, and remove nonessential material. Topology optimization informed me about which parts of the blank experience little to no stress and subsequently can be removed, without risking failure under the expected loads. The Anti-Roll Bar is only a feature of the car’s rear, and hence, only the rear rockers have been designed to link to it.
Front Rockers CAD
Rear Rockers CAD
The interfacing components were assembled with the rockers to ensure proper compliance throughout the full, expected range of motion.
Front Rockers Assembly
Back Rockers Assembly
Hubs
The process of designing the hubs similarly begins with a stepped shaft with dimensions that correspond to where interfacing components attach. Then, the design undergoes an iterative cycle of shaving down material and checking the Finite Element Analysis (FEA) to ensure the stress does not exceed the material’s strength. From the previous year’s design, the size of the bearings on the front hub has been decreased, because we found smaller ones that can sustain the same loads. This change aims to reduce the weight of the hubs.
Front Hubs CAD (View 1)
Front Hubs CAD (View 2)
The main difference between the front and rear hubs is that the rear hubs interact with the half shaft, which is transmitting torque through a tripod bearing.
Rear Hubs CAD (View 1)
Rear Hubs CAD (View 1)
Here’s a close-up of some of the hubs’ features.
Bearing surfaces (bearings are fitted onto uprights, allowing rotation of the wheels)
Holes for magnets (used to measure wheel speed)
Wheel pin holes (pins constrain rotation of hubs to that of wheel centers)
Wheel nut threads (castle nut screws into threaded shaft, securing wheel centers to hubs)
Hubcap (clamps onto the bearings and secures hubs onto uprights) holes
Tulip geometry (tripod bearing exerts torque from half shaft, rotating the hubs)
Floating pin surfaces (floating pins transfer braking force from the brake discs to the hubs)
Validation
Both the rockers and hubs are made of Aluminum 7075-T6, which is known for its high strength and low density. Finite Element Analysis (FEA) was performed to ensure that the components wouldn’t fail under the expected forces. For the rockers, the forces were applied in-plane and also, in another test case, 10$^\circ$ out of plane. These non-planar forces account for minor misalignment issues in the assembly process.
Front Rockers FEA
Rear Rockers FEA
A minimum factor of safety of 2 was achieved for both the front and rear rockers. For the hubs, the force exerted by the ground onto the wheel, and subsequently, the hubs was modelled. This force was applied in five different orientations, 0$\circ$, 15$\circ$, 30$\circ$, 45$\circ$, and 60$\circ$, to account for the rotation of the hubs. Each orientation was prescribed their own independent FEA. The hubs are radially symmetric, so testing for orientations between 0$\circ$ and 60$\circ$ is sufficient. In another test case, the braking torque is applied onto the floating pin surfaces. Lastly, the torque from the half shaft is applied to the tulip-shaped hole in the rear hubs. The wheel forces, braking torque, and drive torque were validated independently first and then in combination to simulate the worst case — when the car is cornering, braking, and accelerating all at once. The independent test for the wheel forces is shown below.
Front Hubs FEA
Rear Hubs FEA
A minimum factor of safety of 1.6 was achieved for both the front and rear hubs.
Manufacturing
Rockers
I used Siemens NX CAM software to generate the G-code for a CNC router to machine the rockers out of an aluminum blank.
Aluminum Blank Machined by 5-axis CNC Router
The parts were then cut out from the blank using a bandsaw and cleaned up on a belt sander.
Finished Front Rockers
Finished Rear Rockers
Hubs
The hubs were manufactured out-of-house due to their complexity.
Finished Hubs
Results
Suspension (Hubs Hidden by Hubcap and Uprights)
Both the rockers and hubs performed without flaw. No failures occurred during the racecar’s drives. The front rocker design was 70.7% lighter than that of the previous year’s. The rear rocker design was 12.3% lighter. The front hubs were 10.2% lighter due to the reduction of the bearing size, and the rear hubs were 2.4% lighter. Overall, the weight reduction benefits the racecar’s performance by increasing the acceleration it’s capable of.
